Optically addressed spatial light modulators (SLMs) are devices having a local response to some incoherent incident optical beam that can be used to control the behavior of a second, generally coherent, optical beam. Because of this function, optically addressed SLMs are becoming increasingly important as input/output devices in sophisticated optical signal processing and computing systems.
A common feature of many optical signal processing systems is the need for a high performance optically addressed SLM to perform parallel temporal integration on an array of optical signals. Although a variety of different SLMs have been used in such systems, SLMs based on photorefractive holography are particularly promising in applications requiring high resolution, sensitivity, and dynamic range. Photorefractive holography, however, usually involves the creation of a periodic intensity grating produced by interference of a coherent input beam with a coherent reference beam within the holographic medium. This process must be modified if incoherent input beams are to be used to create the holograms.
The Photorefractive Incoherent-to-Coherent Optical Converter (PICOC), which is well known in the art, uses coherent plane wave beams to write and read out a photorefractive hologram. The input image is provided in a separate beam that locally erases the photorefractive grating. The image beam may be incoherent because it does not interfere with the coherent writing and reading beams. The process spatially modulates the diffracted output depending on the local intensity of the erasing beam. However, the PICOC must be operated in the weak signal regime since strong input signals, with intensities near those of the writing and reading beams, give rise to saturation of the output response. As a result, the PICOC represents input signals as a weak modulation of a strong output beam, and the output tends to be low in contrast. Although the ability to receive and process incoherent input images is desirable, most optical image processing systems function best receiving the high contrast associated with coherent light holography. Thus, there is a need for an improved, optically addressed SLM that achieves high resolution and sensitivity with incoherent, information-bearing light inputs.